The present invention is generally directed to series wound motors for use in power tools, and more particularly to such motors that have a regenerative braking capability.
Series wound electric motors that are used in many applications, including electrical power tools have an operating characteristic that is generally considered to be undesirable, namely, the motors tend to exhibit a relatively long coast-down time after the power supply voltage to them has been switched off. In some applications such as circular saws, for example, brake stopping time may be relatively long after the motor has been switched off due to the inertia of the motor armature, the gearing, the shaft and the circular saw blade. This coast-down time is not only a nuisance for power tool users, it presents a potential risk of injury to an operator who is careless or impatient when using such a tool.
Because the coast-down characteristic has been recognized as a problem for decades, there have been attempts to provide mechanical as well as electrical braking systems for such motors. Known electronic braking systems for universal motors employ some type of regenerative braking technique, which is based upon the fact that all motors can exhibit generator characteristics. When a tool is switched off, therefore, the motor behaves similarly to a generator in that power is generated for as long as the armature keeps spinning and a magnetic field from the stator exists. Universal motors employing wound fields on the stator are not easily braked since the magnetic field quickly collapses upon switch-off, which is why regenerative braking is employed. At switch off, there is only enough residual magnetism to allow generator action to occur for a short time. However, if all or a portion of this initial generated power is fed back into the stator coils, by placing the coils across the generator output, the magnetic field of the stator is xe2x80x9cregeneratedxe2x80x9d for as long as the tool keeps rotating. In placing the field coils across the armature to allow regeneration of the magnetic field, the field coils themselves act as the load which results in the braking torque. It is common for a resistive element to be placed in series with the field coils to limit high current spikes, adjust braking time, and improve the longevity of internal components.
One important aspect regarding regenerative brakes deals with the construction of the field coils or with the connection of the field coils to the armature. For regenerative braking to occur, the polarity of the magnetic field must remain the same for braking as it was for normal running (or motoring). This is achieved most commonly with either of two techniques: by interchanging the connections between the field and armature at switch off, or by using a second set of field coils at switch off for the purposes of braking, wherein this second set of coils is oppositely wound in the same stator slots as the normal motoring coils.
However, regenerative braking has its disadvantages. Although it was previously stated that all motors exhibit generator characteristics, most motors are poor generators. This is primarily due to the fact that motors and generators are constructed differently. Motors require a lead angle relative to a geometric neutral position against the direction of rotation whereas generators require a lead in the direction of rotation relative to the neutral position. In this regard, the geometric neutral position is a straight line that lies perpendicular to the field poles. This may differ from a magnetic neutral position which is the north/south magnetic axis of the armature which results when power is applied to a pair of brushes spaced 180xc2x0 apart contacting the commutator.
If the same lead angle is employed during running and braking, there can be drastic consequences when a motor with a back lead is forced to brake using conventional regenerative techniques. At tool switch off, when the regenerative action occurs for accomplishing braking, a huge spark is often witnessed at the brush/commutator interface. This spark damages the brushes and the commutator and reduces the life of the motor. The spark occurs because the motor is optimized to run as a motor with a back lead and is then switched run as a generator which requires a forward lead.
A compromise may be implemented in a regenerative brake design, by lessening the motor lead in order to obtain acceptable braking. However, by lessening the motor lead, motor performance is sacrificed. It may also be necessary to ensure a stronger brush pressure on the commutator in order to achieve acceptable braking. But again, this has adverse effects on motor performance and motor life.
An electric motor having a regenerative braking capability is described which has several embodiments, each of which utilize two pairs of brushes, with each pair having individual brushes positioned on diametrically opposite sides of the commutator. One pair is positioned for preferably optimally running the motor while the other pair of brushes is used solely for braking purposes.
For normal motor or running operation, a motor switch assembly interacts with an activating mechanism to physically lift the pair of braking brushes from the surface of the commutator and also ensures that the running brushes come into contact with the commutator.
When the motor switch assembly is switched to stop the motor, the switch assembly causes the activating mechanism to first lift the running brushes from the surface of the commutator and then place the braking brushes onto the commutator surface.